1. Field of the Invention
The present invention relates to an electrophotographic copying process and device in which a photoconductive surface is electrostatically charged and exposed to an information-carrying original to produce a latent charge-image. The latent charge-image obtained is developed by means of a developer liquid to produce a visible toner-image. Excess developer liquid is removed by a metering element which is located a short distance from the photoconductor surface and to which a bias voltage of the same polarity as that of the charged photoconductive surface is applied. The toner-image is transferred from the photoconductive surface onto a copying material, is fixed thereon, and the photoconductive surface is cleaned and/or discharged.
2. Discussion of Related Art
German Offenlegungsschrift No. 3,018,241 discloses a method for removing excess developer liquid from a photo-conductive surface on which an electrostatic charge-image has been developed. The developer is composed of a suspension of charged toner-particles in an insulating developer liquid. In the disclosed method, a drying element, in the form of a squeegee-roller or absorbent roller is brought into contact with the photoconductive surface. This squeegee-roller or absorbent roller is maintained at a potential having a polarity which is identical to that of the charge on the charged toner particles. In addition, the relative motion between the photoconductive surface and the squeegee-roller or absorbent roller is controlled to be zero in the contact region. The cylindrical surface of the squeegee-roller or absorbent roller is composed of an elastomeric material exhibiting a Shore-A hardness of less than 45 and a resistance value of less than 10.sup.9 Ohm.cm. The photoconductive surface is located on a drum which runs counterclockwise past a metering roller or stripping roller which is capable of limiting the quantity of liquid remaining on the photoconductor after the development of the latent charge-image. This metering roller or stripping roller does not touch the developed charge-image, so that neither streaks nor distortions are produced. After passing the metering or stripping roller, a layer of developer liquid with a thickness of between 10 and 15 .mu.m remains on the photoconductor surface and the surface of the drum passes over the squeegee-roller roller or absorbent roller. The bias voltage on the squeegee-roller or absorbent roller produces an electric field which holds the toner firmly on the photoconductor surface. The bias voltage has the same polarity as the toner particles in the developer liquid; thus the developed image remains adhered to the photoconductor surface without producing streaks or smears, and without transfer of toner onto the squeegee-roller. After running past the squeegee-roller, the layer of liquid developer remaining on the photoconductor surface is reduced to a thickness of 2 to 3 .mu.m, so that, overall, the thickness of the layer of developer liquid on the photoconductor is reduced to approximately a fifth of the initial value.
Apart from advantages, such as high resolution and low energy for fixing the copies, which the liquid developer method has as compared to the dry-developing technique, the liquid developer method also has the disadvantage that the copies have to be heated following the transfer of the toner image from the photoconductor surface onto the copying material. This is due to the fact that residual developer liquid remains on the copies and must be evaporated during the operation of fixing the copies by heating. As a result, large quantities of developer liquid are lost and must be continually completed in the copying apparatus. Also, due to this evaporation, the air in the vicinity of the copier becomes undesirably laden with evaporated developer liquid. Although the customary developer liquids are not toxic per se, since, in the majority of cases, they are aliphatic hydrocarbons such as i-decane in which the charged toner particles are dispersed, this large loss of developer liquid is undesirable because it leads to a certain level of environmental pollu- tion.
In the present state of the art as described, for example, in U.S. Pat. No. 3,907,423, the loss of developer liquid following the development of the charge-image on the photoconductive layer by electrophoretic deposition of charged toner particles is reduced by reducing the projecting excess thickness of the layer of developer liquid. This reduction in thickness is carried out before the toner image is transferred to the image receiving material by a stripping roller rotating counter to the rotation of the photoconductor. The stripping roller rotates at a high peripheral speed counter to the movement of the photoconductive layer at a distance of only approximately 50 .mu.m from it. The toner-images deposited on the photoconductive layer are not smeared; however, only a portion of the projecting quantity of developer liquid is removed, so that moist copies are discharged.
The stripping roller or metering roller is separated from the photoconductor surface by a gap which has a width of 0.05 to 1 mm. As a result of the contrarotation of the metering roller at a peripheral speed which exceeds that of the photoconductive drum, the developer liquid is divided into two oppositely-directed flows in the metering gap between the photoconductor and the metering roller. One of these flows is sheared-off by the metering roller and removed by a wiper-blade located downstream. In the case of a metering gap of 100 .mu.m, approximately 0.2 g of developer liquid is lost to one DIN A4 copy, so that the starting assumption can be made, on account of the linear relationship between the loss of developer liquid and the metering gap, that in the case of a gap of 50 .mu.m the loss will amount to approximately 0.1 g of developer liquid.
The metering roller is retained in insulated bearing plates and a bias voltage of approximately 300 V develops on the roller as a result of induction. The image areas on the photoconductor surface are at an electrical potential of 900 to 950 V, while the image-free background areas are at an electrical potential of approximately 150 V. Since the metering roller is at an electrical potential which is lower than that of the image area and higher than that of the image-free areas on the photoconductor, the toner flows from the image-free areas to the image areas and is firmly retained at the latter.
Arrangements can also be made to apply a bias voltage to the metering roller by means of a source of direct voltage. This bias voltage is adjusted so that it is lower than the electrical potential at the image areas, and higher than the electrical potential at the image-free areas on the photoconductor surface. The sole effect of applying the bias voltage is a reduction in the background-shading of the copies and the achievement of a high-contrast image.
Up until very recently, repeated attempts have been made to remove excess developer liquid from the photoconductor surface after the development of the electro-static charge-image to effect a further reduction in the loss of developer liquid to the copies. In these attempts both absorbent rollers made of a foamed polymer with open pores and squeegee-rollers have been employed. The squeegee-roller technique is described, for example, in U.S. Pat. No. 3,299,787. This patent discloses the use of a squeegee-roller with an associated cleaning element for removing the excess developer liquid from a photoconductive belt.
It is not feasible to reduce the loss of developer liquid by increasing the peripheral speed of the metering roller due to the fact that the loss decreases only asymptotically as a function of the peripheral speed. Thus, the peripheral speed required to reduce a noticeable loss would be attainable only with great difficulty in conventional copiers. In addition, a noticeable reduction in the gap between the metering roller and the photoconductor-surface to less than 50 .mu.m, which should result in a reduction in the loss of developer liquid, cannot be achieved due to the precision-engineering tolerances relating to the straightness of the photoconductive drum and of the metering roller. Compliance with specified precision-engineering tolerances becomes more difficult as the lengths of the photoconductive drum and of the metering roller are increased, in order, for example, to produce copies in the DIN A1 format. On account of greater bulging of the drums and metering rollers, the aim must be to employ gaps in excess of 50 .mu.m for the production of large area copies, while at the same time attempting to reduce the loss of developer liquid.